Hydrophilic acrylic fibers having a water retention capacity of at least 10% are obtained according to DE-PS 2 554 124 by adding to the spinning solvent (dimethylformamide) from 10 to 50% by weight, based on the solvent and solids content, of a liquid (preferably glycerol or tetraethyleneglycol) which boils at a temperature higher by 50.degree. C. or more than the boiling point of the spinning solvent and is readily miscible with the spinning solvent and water and a non-solvent for the polymer, and spinning the resulting spinning solution at relatively low spinning shaft temperatures which are 5.degree. to 30.degree. C. above the boiling point of dimethylformamide.
The non-solvents used for polyacrylonitrile are preferably polyhydric alcohols or mono-or polysubstituted alkyl ethers or esters of polyhydric alcohols such as diethylene glycol, triethylene glycol, tetraethylene glycol or high boiling alcohols such as glycerol.
The solvent used for polyacrylonitrile is preferably dimethylformamide (DMF) but dimethylacetamide may also be used.
The acrylic fibers should contain .gtoreq.85% by weight of acrylonitrile, preferably .gtoreq.92% by weight of acrylonitrile in the acrylonitrile (co)polymer. For textile uses, the copolymer content has a value below 15% by weight, down to about 2% by weight, in particular from 8 to 3% by weight.
A water retention capacity of about 35% (30 to 40%) has been found optimal for ensuring good wearing comfort of textiles produced from such fibers which are usually worn close to the skin; see W. Korner: Chemiefasern/Textilindustrie 29/81 (1979), pages 452 to 462 and 31/82 (1981), pages 112 to 116.
For obtaining a high water retention capacity (WR) of about 35% by weight in hydrophilic acrylic fibers with a core/sheath structure, it is necessary to use a relatively high proportion of non-solvent in the system of polyacrylonitrile/non-solvent/solvent, i.e. as a rule the PAN-solid/non-solvent ratio should not be more than about 1.3/1 for obtaining the high pore volume required for producing such a high water-retention capacity. High non-solvent contents, however, entail problems, inter alia in attempts to wash out the non-solvents as thoroughly as possible and stabilize the pores of the fibers.
When such hydrophilic, porous core/sheath fibers. are dyed, it must be remembered that the pore structure reinforces the light scattering on the fibers Dyed hydrophilic acrylic fibers having a water retention capacity of about 35% therefore have less depth of colour and brilliance due to the increased light scattering than a conventionally produced commercial acrylic fibers dyed with the same formulation (see D. Heinkes, Chemiefasern/Textilindustrie 36/88 (1986), pages 911-912.
Due to the high wearing comfort of textiles containing hydrophilic PAN fibers, they are used, as already mentioned, for sports and leisurewear clothing, especially of the kind worn close to the skin, and the textiles are often mixed, especially with cotton and wool. One particular problem of these textiles is the wet fastness of coloured articles; on account of the hydrophilic character of these textiles, they are worn close to the skin as perspiration absorbent garments and therefore frequently exposed to washing at 60.degree. C. Under these washing conditions, which are more severe than those commonly used for acrylic fibers, the dye is liable to bleed from the acrylic fibers and stain accompanying fabrics and fibers such as wool, cotton, viscose, polyamide or polyester. This behaviour of fibres is hardly acceptable in practice.
It has now surprisingly been found that the PAN solids/ non-solvent ratio of about 1.3/1 hitherto required for obtaining a water retention capacity of about 30 to 60% in such hydrophilic core/sheath acrylic fibers can be very considerably shifted in favour of the PAN solids content (i.e. the non-solvent content may be reduced) without thereby lowering the water retention capacity to below 30% and without the usual unwanted staining of coloured articles under washing conditions at 60.degree. C. if the hydrophilic fibers are subjected to a special after-treatment combination, in particular a highly intensive washing treatment with removal of the (non) solvents to reduce them to certain values, stretching in steam followed by fixing by a steam treatment and a mild, low tension drying of the fibers in the form of cut fibre flock (not fiber tows).
This for the first time enabled the PAN solids/non-solvent ratio to be raised to 3.25/1 without the water retention capacity thereby falling to below 30-35%. This resulted in considerable savings of cost in the manufacturing process, the use of smaller quantities of non-solvent, much more favourable (minimal) residual solvent contents in the hydrophilic fiber, substantially improved dye fastness (less bleeding) and a very stabilized pore volume in the hydrophilic fiber. Other advantages will be mentioned in the course of the description.
In spite of the fibers presumably having a smaller pore volume due to the reduction in the quantity of the non-solvent component which produces the pore structure, it is possible, by means of the more intensive washing out of the non-solvent component, stretching in steam and fixing of the pore volume with saturated steam followed by a mild, low tension drying of the cut fibers at only moderately high temperatures, (e.g. on a screen belt drier), to preserve the core volume in the fibers from the spinning process to the various after-treatment stages and beyond and to stabilize this pore volume and thus provide the means for moisture absorption, in other words to produce a high water retention capacity.
When the fibers are being dried under relatively mild conditions, they should be present in the form of flocks (cut fibers) or non-woven webs, i.e. as cut fibers, and not as folded, crimped spinning tows. It has been found that when the fibers are present in this flock form, a uniform sheet is obtained on the screen belt drier in the drying process and hence the fibers can be heated through and dried uniformly under mild conditions. If, on the other hand, the fibers are dried as crimped, folded, endless tows and placed on the drying surface in layers of the usual thickness employed for this process (e.g. 10 kg/m.sup.2 of effective drying surface), differences in density and moisture (so-called moisture nests) frequently occur between the outer and inner layers of the tows As a result, non-porous, glossy tow sections are obtained side by side with porous, matted tow portions due to uneven heating and mass distribution on the (screen belt) drier (see Example 3). If, instead of the fibers being applied in thick layers (e.g. 1.26 kg/m.sup.2 of effective drier surface in Example 3) the fibers are spread more thinly (e.g. up to 0.6 kg/m.sup.2) and pass through the drier under mild conditions, these differences in the quality of the fibers can be greatly reduced or even removed. Such low covering densities, however, entail a high consumption of energy and are undesirable for economical reasons and generally unacceptable.
For obtaining the hydrophilic, porous polyacrylonitrile threads with a core/sheath structure according to the invention which are spun with smaller quantities of non-solvent, the conditions of the process had to be revised/limited as described above so that the (primary) pore structure obtained would not collapse or be impaired in the after-treatment stage following the dry spinning process. At no stage of the after-treatment process (except in the steam crimp and its crimp region) may a treatment temperature above 125.degree. C. be employed in steam, and the temperature is preferably not above 109.degree. C. in steam.
It has been found particularly advantageous to integrate the after-treatment of such hydrophilic acrylic fibres having a core/sheath structure with a continuous spinning and after-treatment process such as that first described, for example, in EP 119 521 for non-hydrophilic PAN threads, but even here, modifications have become necessary.
Compared with previously disclosed processes, very much higher water retention capacity values are obtained according to the invention for the same proportions of non-solvent. This means that for the technically important range of WR=30 to 60%, preferably 33 to 40%, only a very much smaller proportion of non-solvent is required. This means that the primary pore structures obtained after spinning can be much more easily stabilized in the course of the after-treatment stages and the amount of solvent and non-solvent still present in the threads is less by a factor of 3 to 5 (or more) after the first intensive washing stage than in the previously known processes and amounts to less than 2% by weight, preferably less than 1% by weight.
One unexpected result was the cross-sectional form of the threads obtained by the new process. The cross-section of the threads has become much more uniform and substantially oval (see FIG. 1 corresponding to Example 1) as compared with the much more non-uniform, ill defined, partly collapsed thread cross-sections obtained by the previous process (see Example 2, FIG. 2). These more uniform, rounder cross-sectional shapes also have their effect in yarns and textile webs produced from the fibers in that these products have a much improved, softer handle (as well as improved fastness properties) whereas the threads obtained by the processes hitherto known in the art were "rougher" and gave rise to more "scratchy" yarns.
The fibers obtained by the process according to the invention resemble the fibers known in the art in having a core/sheath structure with a porous inner core (see FIGS. 3 and 4).
The new procedure according to the invention has not only enabled the manufacturing costs to be considerably reduced on account of the much reduced non-solvent content but has also markedly improved the wet fastness of the hydrophilic core/sheath acrylic fibers.